Nocardia, an organism closely related to Mycobacterium, can cause fatal infections in man. Little is known about the mechanisms controlling nocardial invasion, infection, and disease production. In contrast, a great deal of work has been done on mycobacterial diseases. It has been shown with the mycobacteria that much of the organisms' effect on the host as well as its virulence resides in the chemical makeup of the cell envelope. Therefore, I propose to study the in vivo and in vitro chemical composition of nocardial cell walls. I shall explore the effect of the host as well as the environment on these components in order to determine how they affect the host-parasite relationship. I shall compare and contrast the nocardial interactions with alveolar and peritoneal macrophages and investigate the effects of immunosuppressants on macrophage function in nocardial disease. At the same time I shall study the chemical composition of nocardial cell walls in vitro and in vivo in order to determine their specific role(s) in the host-parasite response. By using nocardial strains that vary from non-infectious to fully virulent forms, I hope to establish more completely the factors contributing to nocardial virulence. BIBLIOGRAPHIC REFERENCES: Beaman, B. L. 1976. Possible mechanisms of nocardial pathogenesis. In: Biology of the Nocardia. G. Brownell, M. Goodfellow, and J. Serrano, Editors. pp. 386-417. Academic Press, London, Great Britain. (In press). Beaman, B. L. 1977. The in vitro response of rabbit alveolar macrophages to infection with Nocardia asteroides. Infect. Immun. 15: (In press).